In the age of Twitter, how much control do you have over your own words? Authorities are trying to use an Occupy protester's tweets against him in court, but Twitter's fighting back in a case that raises big questions about online privacy.

Today, Twitter asked a judge to quash a subpoena requesting three months of public tweets and account information belonging to Occupy Wall Street protester Malcolm Harris. The 23-year-old was charged with disorderly conduct last December, when he, along with more than 700 other Occupy Wall Street protesters, blocked the Brooklyn Bridge during a march. (You'll also remember Harris as the guy who spread false rumors that Radiohead was playing down at Zuccotti Park.) In February, the Manhattan District Attorney's office faxed the subpoena to Twitter HQ. Prosecutors have said they want Harris' old tweets to counter his "anticipated defense"—that he had been led onto the bridge by NYPD officers.

But Harris filed a motion to quash the subpoena on the grounds that seeking a three-month record of his online life is overkill in prosecuting his disorderly conduct charge, a violation about as serious as a parking ticket. The Electronic Freedom Foundation called it a "government fishing expedition." Still, a judge denied Harris' motion to quash, arguing he had no legal standing to challenge the subpoena because he doesn't own his tweets—Twitter does. Once you post a tweet, the judge argued, it's no longer yours.

But today, Twitter forcefully rejected that idea. In its own motion, Twitter argues that its users retain the rights to any content they post to Twitter, which means users like Harris can defend themselves against subpoenas of their accounts. Twitter also argues the DA's warrantless demand for user information violates the Fourth Amendment.

"This is a big deal," writes the ACLU in applauding Twitter's admirable challenge. "Law enforcement agencies—both the federal government and state and city entities—are becoming increasingly aggressive in their attempts to obtain information about what people are doing on the Internet. And while the individual Internet users can try to defend their rights in the rare circumstances in which they find out about the requests before their information is turned over, that may not be enough."

But this fight is over tweets that Harris had posted publicly, viewable to everyone on the internet when they were up. As the judge wrote in striking down Harris' motion: "[Harris'] account's Tweets were, by definition public. The defendant had knowledge that Twitter was to instantly distribute his Tweets to Twitter users and non-Twitter users, essentially anyone with Internet access."

So what is the big deal?

Harris' case is about just how much control we have over our privacy as our online lives take place increasingly on third-party platforms like Twitter. Harris may have posted his tweets publicly at first, but he deleted many of them soon after the bridge incident. Imagine if Harris had posted his tweets on pieces of paper tacked to a physical bulletin board, then took them down and hid them in his house. The DA couldn't just barge in without a warrant because they suspected he'd once posted incriminating messages to the board. It should be the same for his deleted tweets, even if they live on Twitter's server. Our ability to hide what was once public is just as important to privacy as controlling what's private in the first place.

This gets at a bigger problem: the law and technology companies are currently incapable of dealing with any concept of privacy online that goes beyond the simple dichotomy of public vs. private. (Some are trying.) If you don't engage confusing, often-useless privacy controls, your only choice is to act in front of the entire internet, and leave a record forever.

But increasingly, people exist in spaces that exist somewhere in between the public and the private—and that's often where we have the most control over our privacy, too. Many teens, for example, use complicated strategies like deleting all of their wall posts and status updates on Facebook soon after they're posted. This allows them to interact freely without leaving an obvious paper trail that might spark drama later on. Occupy Wall Street protesters, who have been subjected to oppressive surveillance and harassment, might reasonably use similar strategies as they try to balance the risks and opportunities of organizing transparently online.

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At the heart of this case is the question: Should everything we ever post to social media be considered public and fair legal game, regardless of how we act on it in the future? We've grown used to the idea that once you put something on the internet, it's fair game forever. The fact that the government so eagerly espouses this view when it comes to Occupy Wall Street protesters should make us think twice about it.